1. Field of the Invention
This invention relates to an optical waveguide structure in which an electric field may be applied to achieve electro-optic effects, and has particular application to optical fibres.
2. Related Art
It is well known that lithium niobate changes its optical characteristics in response to an applied electric field and can be used as an electro-optic modulator or a non-linear optical element, particularly in an optical fibre. However, it has a number of drawbacks, particularly high coupling losses when coupled to a standard fibre and a low-photorefractive damage threshold, which have prompted an investigation of the electro-optic effects of glassy materials and in particular silica.
Thermally assisted poling of silica has been known to induce electro-optic coefficients in both bulk silica and optical fibres, and reference is directed to L. Li & D. N. Payne "Permanently-Induced Linear Electro-Optic Effect in Silica Optical Fibres, Dig. Conf. Integrated and Guided Wave Optics, 1989 OSA, Paper TuAA2-1 (1989). However, the coefficient induced in this way is not sufficiently high to allow practical devices to be constructed.
It has recently been found that germanosilicate fibre, which is photosensitive to u.v. light, can be photo-excited with incident u.v. radiation to produce an electro-optic coefficient comparable to that of lithium niobate. Reference is directed to T. Fujiwara, D. Wong, Y. Zhao, S. Fleming, V. Grishina & S. Poole, "UV-Excited Poling and Electrically Tunable Bragg Gratings in a Germanosilicate Fibre", Postdeadline Paper OFC '95 (Feb '95). The u.v. technique has a significant further advantage over thermal poling in that it permits the writing of gratings and other structures in the fibre.
In order to achieve a sufficiently high applied field for the fibre, it is has previously been proposed to modify a conventional germanosilicate fibre which has a Ge doped core of relatively high refractive index surrounded by SiO.sub.2 cladding of relatively low refractive index, so as to include longitudinal apertures in the cladding to receive electrodes in the form of metal wires running generally parallel to the core on opposite sides. By placing the electrodes close to the core, within the cladding, a sufficiently high field can be developed across the core in order to induce changes in the refractive index of the core. Reference is directed to S. C. Fleming, T. Fujiwara and D. Wong "UV Excited Poling of Germanosilicate Fibre" OSA '95 Photosensitive non-linearity in Glass waveguides - Fundamentals and Applications, OSA Technical Digest Vol. 22 1995. The fibre was fabricated by milling a pair of holes into the end face of a preform close to its core and positioned diametrically across the core with respect to one another. The preform was then drawn into fibre in a conventional manner so as to form a fibre with a core diameter of 8 .mu.m and a spacing of 18 .mu.m between the apertures that receive the electrodes. The apertures were of a diameter of the order of 70 .mu.m and the electrodes wires had a diameter of the order of 50 .mu.m. The electrode length was in one example 6 cm.
A disadvantage of this structure is that the electrodes need to be inserted into the fibre after formation. It will be seen that the electrode wires are of very small diameter and consequently difficult to handle. Furthermore, because the structures are so small, the electrodes have to be arranged to extend out of the apertures at opposite ends of the device in order to avoid risk of them touching, which therefore requires long connection leads. The entry of the leads in end faces of the fibre makes it very difficult to splice the fibre to conventional optical fibres, so that it cannot be included readily in optical circuits. Conventional fusion splicing could not be used because the heat required causes air in the holes to expand and distort or damage the heat softened glass of the fibre. Also, the holes need to be of a larger diameter than the electrode wires to allow them to be fitted, with the result that they are not held at a fixed distance from the core of the fibre. This can result in a non-uniform field being applied, in use, along the fibre.
Partial removal of the cladding of a fibre has been proposed in U.S. Pat. No. 5,265,178 for the purpose of allowing a doped polymer to be placed close to the optical fibre for modulation purposes, rather than for applying a field to the core. Planar structures with modulating electrodes have been proposed in "Low-loss Strain Induced Optical Waveguides in Strontium Barium Niobate at 1.3 .mu.m wavelength", J. M. Marx et al Appl. Phys. Letts. 66 (3) January 1995, pp 274-276.